In U.S. Pat. No. 3,607,356, it has been proposed to impregnate cellulose-containing materials with an aqueous solution containing an equilibrium mixture of tris(hydroxymethyl) phosphine ("THP") and tetrakis (hydroxymethyl) phosphonium hydroxide ("THPOH") said solution having a pH of about 7 to about 8. In this process, the impregnated material containing from 10 to about 40 percent by weight of the monomer is dried to about 10 to about 20 percent moisture and then treated with gaseous ammonia in an enclosed cabinet to polymerize the resin monomer. The gaseous ammonia treatment step requires from one or two to 6 minutes exposure time depending upon the character, i.e., the weight, fibrous nature, etc. of the treated material. In many textile processs, the materials are processed in equipment operating at high speed in a continuous manner. Accordingly, operations involving processing times of several minutes duration require either static operation or units of a size wherein each relatively long residence times can be obtained. It is, therefore, desired to process such materials in equipment wherein shorter residence times consistent with high speed continuous operations can be obtained in a practical manner. Moreover, the treatment according to the process of U.S. Pat. No. 3,607,356, when used with available ammonia treatment cabinets, has been found to produce finishes on cellulosic materials which often tended to dust and the durability of the finished materials often failed to meet the stringent government standards (Department of Commerce Standard FF-3-71) which require that the treated materials withstand at least fifty home washing and drying cycles. The enclosed chamber for carrying out the ammonia treatment commonly used in this field comprises a series of perforated pipes housed in a box like enclosure having a large opening in the top. The partially dried impregnated material is passed over the perforated pipes through which ammonis gas is forced. The excess ammonia gas is vented through the opening in the top of the enclosure, and discharged into the atmosphere. This venting of considerable quantities of ammonia gives rise to a severe pollution problem. It can thus be seen that the process disclosed in U.S. Pat. No. 3,607,356 not only results in a highly inefficient utilization of ammonia but also is hardly practical for the lighter, open weave, materials and leaves much to be desired when processing heavier and/or close knit materials.
It is known also, as disclosed in U.S. Pat. No. 2,983,623, to cure further polymerizable methylol-phosphorus polymeric material containing at least one free methylol group attached to a phosphorus atom incorporated in a cellulosic material, by exposing said material in the dry state to the action of gaseous ammonia followed by subjecting it to an aqueous ammonia treatment. In this process, the further polymerizable resins disclosed are solutions of reaction products of tetrakis(hydroxymethyl) phosphonium chloride and urea which solutions are relatively strongly acid and are applied in the presence of buffers which adjust the pH of the solutions to a pH within the range of about 3.5 to 4. The impregnated material is thoroughly dried, exposed to ammonia gas for about 5 to 10 or more minutes, and then immersed in aqueous ammonia for about 10 or more minutes to complete the cure of the resin on and in the material. Such a process also requires relatively long time cycles of treatment especially in the aqueous ammonia hence is hardly applicable with modern high speed processing techniques.
The problem of the long time cycles and efficiency of the polymerization has been substantially overcome by the apparatus and process disclosed in U.S. Pat. No. 3,933,122 issued Jan. 20, 1976. In this application an apparatus and process for imparting flame retardance to cellulose containing materials is disclosed whereby materials which have been impregnated with a solution having a pH of from about 7 to about 9 and a tetrakis (hydroxymethyl) phosphonium hydroxide content of from about 0 to about 8 percent and the monomer is polymerized in and on the cellulose material by exposure in an enclosed chamber to an atmosphere containing from about 50 to about 90 percent by volume of gaseous ammonia for about 5 to about 30 seconds. By this process and with this apparatus the monomer is rapidly and effectively polymerized on and in the cellulose containing material, thereby imparting durable flame retardance to the materials in a rapid and efficient manner. Under certain mill conditions it was found that substantial amounts of formaldehyde was formed during the gaseous ammonia exposure step and when the cured fabric was batched in rolls or on trucks immediately upon exit from the ammonia chamber, it was noted that the odor of formaldehyde rapidly developed in the batched processed materials and also that a considerable exotherm was prevalent in the material. In such materials, i.e., when the odor of formaldehyde and/or exotherm was noted, the durability of the flame retardant character was reduced. It is believed that the formaldehyde produced by decomposition of the polymerized or partially polymerized monomer when confined in the material, reacted with the polymer or partially polymerized polymer to form a moisture sensitive reaction product which may deleteriously affect the durable character of the flame retardant treatment.
The process of the present invention represents an improvement over the process of the aforementioned U.S. Pat. No. 3,933,122 whereby the deleterious effect of the action of formaldehyde is obviated and thus a rapid and effective means of imparting durable flame retardance to cellulose containing materials is provided. Further said improved process is more generally applicable with the currently used high speed textile processing equipment and conditions.